1. Field of the Invention
The present invention relates to a chemical mechanical polishing (CMP) apparatus and more particularly, to a CMP apparatus including an apparatus for detecting a polishing end point.
2. Description of the Related Art
With multiple-layered lines employed in highly-integrated semiconductor fabrication, the fabrication operation involves processes of forming thin films having desired patterns on a wafer, and planarizing the wafer using a CMP apparatus before repeatedly subsequently forming additional thin films on the wafer.
A CMP process is performed by contacting the surface of a wafer with a polishing pad including a polishing slurry, applying a predetermined pressure onto the wafer, and rotating the polishing pad and the wafer at a predetermined speed so as to simultaneously perform the chemical and mechanical polishing. Certain problems which need to be addressed in the CMP process are detecting the removal rate of the wafer and determining the polishing end point. In the conventional case, the polishing process is performed within a predetermined process time. The polished state of the wafer is determined by visual inspection of the polished surface of the wafer. Therefore, the polished surface may be often overpolished or it may require a further polishing. Currently, the thickness of the layer on the wafer is directly detected during the polishing process by means which are additionally installed in the CMP apparatus. Appropriate polishing end points are detected on the plural portions of the wafer, so as to improve the thickness uniformity of the wafer, and to improve the stability and efficiency of the apparatus. A method of determining the polishing end point can be obtained from (a) the thickness of the wafer measured by a thickness measurement device during the polishing process, (b) a platen and the changes of load current, voltage, and resistance of a wafer carrier motor during the polishing process, and (c) the irradiation of a laser on the wafer and the reflection from the wafer during the polishing process.
The detection of a polishing end point using an optical sensor is disclosed in the U.S. Pat. No. 6,190,234. The sensor uses a plurality of optical end point detect (EPD) systems including a first optical system and a second optical system having different wavelengths. The plurality of EPD systems are said to precisely and quickly detect the polishing end point through one or more windows located under a polishing table.
As described above, the optical systems are installed in a plurality of locations of polishing regions (for example, center, middle, edge). Then, if polishing end points are detected in every location, the polishing process is complete. Since the time of completion of the polishing process is when the polishing end points are detected in every location, in the case where the polishing end points between the locations of the polishing regions are large, a problem may be caused. In this instance, the polishing end point may be detected late, and the wafer may become overpolished. The portion being overpolished may in turn form a recess, due to the accumulation of a slurry, or may produce defects such as dishing and corrosion, etc. Further, in the case of installing the plurality of EPD systems using the optical system at a plurality of locations, the exact polishing end point is difficult to achieve even when one error occurs on any one location. Thus, this causes the resultant problems described above. Additionally, in the case of detecting a polishing end point from the measurement through the change of load current of a motor, the exact polishing end point remains difficult to determine when a current signal exhibits any significant amount of noise.